G - General Audience : Suitable for all ages. Minimal violence, no sex (explicit or implicit), no scenes of drug use, no offensive language.
PG - Parental Guidance advised : Fics in this category typically contain a moderate amount of violence, some swearing, but no (explicit) sex or drug use.
PG-13 - Parental Guidance strongly advised : May be unsuitable for children below the age of 13. Fairly graphic violence and use offensive language may be present, along with non-descriptive sex scenes / scenes of drug use.
R - Restricted : This implies that readers under the age of 17 should seek parental consent before viewing said material. Large quantities of graphic violence and language is to be expected, and/or explicit sex scenes, portrayal of drug use, horror, etc.
X - No one 17 and under admitted : Generally, in ficdom, restricted to 'adult' fics with intense, hard-core sex scenes, sexually oriented language and/or scenes of excessive an graphic violence.

Other notationWIP - Work in Progress
TBC - To be continued
BL - Boy Love (formerly shounen-ai). Non-explicit Male/male relationships. May or may not contain sex.
M/M or YAOI - Male/male relationships, containing implied or explicit intercourse. (Used as a general handle.)
F/F or YURI - As above, applying to female/female relationships.
Het - Heterosexual - As above, applying to male/female relationships
Non-con - Non-consensual sex. (ie : Rape)
Chara death - Death of a main character. Dark - Character torture and / or death. Physical, psychological and/or emotional abuse.
Violence - Graphic scenes of brutality and violence (inclusive of fight scenes, injuries, and killing)
Language - Gratuitious use of offensive and crude language.

A x B - implies a sexual relationship between A and B, where A is the 'seme' -- ie, the dominant person in the sexual relationship. In heterosexual relationships, the male is always placed first.
A + B - implies a platonic relationship between A and B. The order of characters is relatively less important.
A / B - implies a slash relationship of any magnitude. Order is not important.

Saiyuki Numbering SystemFavored by the lazy, the l33t, or the just plain geeky, the numbering system in Saiyuki is often used in place of actual names. It is based on the phonetics of the character names.
3 - Sanzo ('San' is 3 in Japanese.)
5 - Gojyo ('Go' is 5 in Japanese.)
8 - Hakkai ('Hachi' is 8 in Japanese)
9 - Goku ('Ku' is 9 in Japanese.)
Hence 38 implies a relationship between Sanzo and Hakkai, while 85 implies a relationship between Gojyo and Hakkai, etc.